This invention relates generally to wafer handling equipment and more particularly, it relates to a hotplate offset ring of a unique construction for use in a heat treatment unit of wafer track system so as to achieve a more uniform heat transfer.
As is generally known in the art, a series of processes are required to be performed during the manufacturing of a semiconductor device. One such process is referred to as a "photolithographic or resist process" in which a resist coating is applied to a wafer, the wafer is then exposed, and thereafter the wafer is developed. There are several known types of systems which are commercially available for performing the resist coating/developing processing on a semiconductor wafer. For example, in industrial semiconductor fabrication facilities, there are used normally automated photoresist processing systems sometimes called a "wafer track systems."
Typically, the wafer track system includes a loader station for loading and unloading cassettes containing wafers to be processed, a resist coating station for applying a coat of resist, a heat treatment station and a developing station for developing the exposed wafer and rinsing the developed circuit pattern. The heat treatment station includes a hot plate unit and/or a cool plate unit for controlling the wafer to be at a predetermined temperature.
In the heat treatment station, the wafer is placed on the hotplate in order to undergo conduction baking so as to bake off fluids, such as, for example, solvents used in applying the photoresist to the wafer or liquids used in washing or cleaning the wafer. It is generally desirable that the hotplate used in the baking operation be able to control closely the wafer temperature at every point within the heating cycle. This is achieved currently through heating the wafer by a proximity baking technique in which the amount of heat transferred to the wafer and the rate at which the heat is transferred to the wafer is determined by a controlled gap positioned between the wafer and the hotplate.
At present, this controlled gap is maintained by an offset washer for controlling the gap spacing between the wafer and the surface of the hotplate and screws for holding securely the offset washer to the hotplate. In order to insure uniform heat transfer from the hotplate to the wafer, the wafer must be positioned accurately in the center of the hotplate of the heat treatment unit. In other words, accurate centering of the wafer with respect to the center of the hotplate is critical so that the wafer will rest upon the washer and not on the screws for holding the washer.
Thus, it can be seen that this close proximity baking technique requires the use of a wafer gripper mechanism of some type to perform an accurate and consistent placement of the wafer onto the hotplate so as to prevent the wafer from resting unintentionally on the screwhead. If the wafer indeed becomes off-centered with respect to the hotplate so that it does rest upon the screwhead, then there will be a non-uniform heat transfer which may adversely affect a change of the line width of the circuit pattern of the photoresist in a localized region of the wafer. Further, the non-uniform heating may also result in the non-uniform thickness of the photoresist across the surface of the wafer which will lead to a line width variation after exposure of the resist.
As a result, there exist a need for a way of controlling the gap between the wafer and the surface of the hotplate so as to achieve a more uniform heat transfer. This is perform in the instant invention by the provision of an offset ring of a novel construction for use in a heat treatment station of a wafer track system so as to prevent the possibility of non-uniformities in a gap formed between the wafer and the hotplate.